Measurement chip, microfluidic device and method

ABSTRACT

A measurement chip ( 100 ) is disclosed for use with a microfluidic resistance network ( 20 ) comprising a microfluidic sample preparation stage ( 34, 38 ), a sample outlet ( 42 ) and a waste outlet ( 44 ) both in fluidic communication with said preparation stage. The measurement chip comprises a sample channel ( 104 ) for receiving a sample from said sample outlet ( 42 ), the sample channel comprising measurement means ( 120, 130 ) and having a first fluidic resistance; and a waste channel ( 114 ) for receiving a waste stream from said waste outlet ( 44 ) and having a second fluidic resistance.

FIELD OF THE INVENTION

The present invention relates to a measurement chip for use with amicrofluidic resistance network comprising a microfluidic sampledilution stage, a sample outlet and a waste outlet both in fluidiccommunication with said dilution stage, the measurement chip comprisinga sample channel for receiving a sample from said sample outlet, thesample channel comprising measurement means and having a first fluidicresistance.

The present invention further relates to a microfluidic device havingsuch a measurement chip and microfluidic network.

The present invention yet further relates to a method of manufacturingsuch a measurement chip.

BACKGROUND OF THE INVENTION

In healthcare, there is a trend towards the development of so-calledPoint Of Care (POC) devices, which are small devices, often withdisposable components such as cartridges, that can be used in diagnosisand treatment of patients as an alternative to large and expensiveanalysis equipment.

A widely used diagnostic test is a Full blood count (FBC) test, which isa diagnostic test that is used to measure cellular composition of blood.It may give information about the status of an immune system of apatient, about the ability of the blood to disseminate oxygen and/orabout the ability of the blood to effectively clot. As such, it is afundamental test that is often used as an initial “general purpose”diagnostic tool or as a more targeted monitoring solution. Examples ofcare cycles that include a full blood count as a monitoring tool includeoncology, arthritis and Crohn's disease. As many as 300 million FBCtests are performed each year in the developed world.

Currently, large scale commercial laboratory instruments known ashaematology analyzers are used to automatically perform all measurementsthat comprise the FBC. The high cost and complexity of these devices,coupled to the need for venous blood, means that they are mostly largescale, centralized facilities. There is a clear clinical need forperforming FBC in a near patient setting, particularly for applicationsthat require a full blood count to monitor the progression and/ortreatment of a disease.

Previously, microfluidic point of care devices have been developed whichare capable of measuring individual components of the FBC. In that area,Hb measuring devices, WBC counters capable of performing a white bloodcell differential and platelet count devices, devices which opticallycount and determine size of red blood cells are available. For cellcounting, current haematology analyzers typically employ electricalcoulter counting and/or optical scattering methods to count anddifferentiate white cells and to count and determine size of the redblood cells and platelets.

At the moment only few examples of micro fluidic coulter countertechnologies exist. One example combines a coulter counter with a Hbmeasurement. Another example of counting cells is by flow-throughimpedance spectroscopy. This is a flow cytometry analysis which isespecially suited for a micro fluidic format. This technique is capableof differentiating between lymphocytes, monocytes and neutrophils inlysed blood, and of counting and sizing red blood cells and platelets.

The current “gold-standard” for Hb measurement is the photometriccyanmethaemoglobin (HbCN) method disclosed in Standardization ofhemoglobinometry II, The hemiglobincyanide method, Clin Chim Acta, 1961,6, p. 38-44. This method involves chemical lysis of the red blood cellsand subsequent labelling of all the Hb that these cells release with acyanide ion. The labels produce a defined absorption profile with amaximum at 540 nm. By measuring the optical absorption at 540 nm, theconcentration of Hb can be determined. Furthermore, the high stabilityof HbCN means that it is easy to supply a calibration standard.

The most common red blood cell lysis/cyanide conversion reagent is knownas Drabkin's reagent. Drabkin's reagent contains Potassium Cyanide,which is extremely toxic. This reagent only works for very largedilutions in whole blood (1:251), since red blood cell lysis relies onthe low ionic strength of the reagent to induce osmotic shock. Thislarge dilution causes an inherent imprecision in the method.Furthermore, to measure the optical absorption at 540 nm, very longoptical path lengths of ˜1 cm are required. Finally, in somepathological samples, turbidity can lead to erroneously high absorptionreadings, which in turn will give rise to an incorrect Hb concentration.

To avoid the problems associated with toxicity and turbidity, many otheroptical means of measuring Hb have been developed. A known point of caredevice uses sodium azide to convert the Hb to an azide-coordinated Hbderivative (azidemethemoglobin, HbNs). This method itself lends to shortpath length (0.1 mm) absorption spectroscopy, since dry reagents removethe need for dilution of the whole blood. Two absorbance readings aretaken to determine the HbN3 concentration, i.e. one at the absorptionmaximum (565 nm) and one at 800 nm to correct for turbidity.

For the point of care WBC/Hb counter, a RBC lysis solution has beendeveloped that preserves the WBCs while at the same time labeling the Hbmolecule with imidazole. In a similar way as described above, theoptical absorption of the imidazole labeled Hb species is measured attwo wavelengths, i.e. one at the absorption peak and one to correct forturbidity and scattering effects for the white blood cells. The samesolution may also be passed through a coulter counter to perform thecell count.

Another known lysis/Hb conversion reagent is based on sodium laurylsulphate/sodium dodecyl sulphate (SLS/SDS). The SDS lyses all the bloodcells and labels the Hb to get an SDS-coordinated derivative. Since SDSis a surfactant molecule, turbidity correction is not necessary and so asingle absorption reading at 535 nm is taken to determine the Hbconcentration. This method is designed for high dilutions of Hb, so theinherent imprecision present in the HbCN measurement is still present inthe HbSDS one.

All the above described devices and techniques are capable of performingspecific measurements from a finger-prick of blood. However, none of theabove described devices and techniques are capable of measuring allparameters that are required for an FBC in a single POC measurement.Recently, a micro fluidic device capable of performing a FBC in a singlePOC measurement has been disclosed in WO 2010/086786. This microfluidicdevice comprises a two sample preparation stages, one for diluting aportion of a blood sample with a lysis agent for a white blood cellcount and a quench solution and providing the diluted portion to animpedance measurement means and a second dilution stage for diluting afurther portion of the blood sample with a diluent for haemoglobinmeasurement and providing the diluted further portion to a measurementmeans for determining properties of red blood cells, such as RBC count,HB count and platelet count. The diluent is fed to the blood sampleseveral times to obtain a high dilution ratio. Consequently, only afraction of the RBC count sample is used for the actual RBC count, withwell over 90% of the various dilution stages being fed to waste.

It is of paramount importance that the flow rates through such amicrofluidic device are well-defined in order to achieve accuratemeasurement results of the FBC. Such flow rates may be controlled usingseparate pumps for each fluid stream, but this is rather costly.Alternatively, the flow rates may be well-defined (i.e. tuned) at thedesign stage of the microfluidic device by tuning the dimensions (i.e.the fluidic resistance) of the fluid channels forming the microfluidicnetwork. As the feature sizes of the microfluidic network are typicallylarger than the feature sizes of e.g. the impedance measurement chip, itis easier and therefore more cost-effective to manufacture themicrofluidic network and the measurement chip in separate processes.

This, however, complicates the tuning of the microfluidic device. Forinstance, as only part of the diluted blood sample is fed to themeasurement chip, the remainder of the diluted blood sample is typicallyfed to waste, as previously explained. Due to the larger feature sizesof the microfluidic network compared to the impedance measurement chip,the waste channel in the microfluidic network typically has to comprisea fluidic resistance matching element to ensure that ratio of thefluidic resistance of the waste channel and the measurement channelthrough the impedance measurement chip is well-defined and comparable.

It has however been found that this matching element does not alwaysachieve satisfactory tuning. This is because the tolerances of themanufacturing process of the microfluidic network are independent of thetolerances of the manufacturing process of the impedance measurementchip, such that the required dimensions of the matching element cannotbe accurately predicted.

SUMMARY OF THE INVENTION

The present invention seeks to provide a measurement chip for use with amicrofluidic resistance network in which the flow rate can be moreeasily controlled.

The present invention further seeks to provide a microfluidic devicecomprising such a measurement chip.

The present invention also seeks to provide a method for manufacturingsuch a measurement chip.

According to an aspect of the present invention, there is provided ameasurement chip for use with a microfluidic resistance networkcomprising a microfluidic sample preparation stage, a sample outlet anda waste outlet both in fluidic communication with said preparationstage, the measurement chip comprising a sample channel for receiving asample from said sample outlet, the sample channel comprisingmeasurement means and having a first fluidic resistance; and a wastechannel for receiving a waste stream from said waste outlet and having asecond fluidic resistance.

The present invention has been based on the realization that balancingof the flow rate of a sample from the microfluidic resistance networkthrough the measurement chip can be much better controlled when thewaste stream is also fed through the measurement chip. This is becauseboth the sample channel and the waste channel exhibit the same reductionin feature size, and are manufactured in the same process, such that thevariation in sample and waste channel dimensions becomes much smaller,thus yielding a more predictable and reproducible tuning of the flowrates through the measurement chip. This has the further advantage thatthe matching element in the waste line of the microfluidic resistancenetwork may be omitted, thereby reducing the manufacturing complexityand cost of the microfluidic resistance network as the matching elementtypically has to be very large or very small dimensions to achieve thenecessary fluidic resistance. To this end, the ratio between the firstfluidic resistance and the second fluidic resistance is typicallypredefined to ensure the correct flow rate through the microfluidicmeasurement channel of the measurement chip.

In a preferred embodiment, the respective dimensions of the samplechannel and the waste channel are the same. This minimizes thevariability in the manufacturing tolerances of the sample channel andthe waste channel. As the fluidic resistance scales with 1/d⁴, where dis the hydraulic diameter of the fluidic channel, it is important tocontrol (variations in) the dimensions of such channels as well aspossible.

In an embodiment, the measurement chip further comprises a sample inletand a sample outlet, wherein the sample channel extends between thesample inlet and the sample outlet; and a waste inlet and a wasteoutlet, wherein the waste channel extends between the waste inlet andthe waste outlet.

In another preferred embodiment, the sample outlet and the waste outletare the same. It has been found that the sample channel and the wastechannel may share the same outlet without affecting the respectivefluidic resistances of these channels. This has the advantage that themanufacturing cost of the measurement chip can be reduced as one lessoutlet needs to be provided.

In yet another embodiment, the measurement chip comprises a plurality ofsaid waste channels that preferably each have the same dimensions as thesample channel. This is for instance advantageous when the flow ratethrough the waste channel is tuned to be an integer multiple of the flowrate through the sample channel, such that the measurement chip mayinclude said integer number of waste channels to implement the requiredflow rate in a controllable manner.

Said waste channels may extend between the same inlet and the sameoutlet, which has the advantage of reduced manufacturing cost.

Preferably, the measurement chip is a glass chip. Glass chips can bemanufactured in a cost-effective manner and with excellent control overthe dimensions of the fluid channels through the glass, e.g. the samplechannel and the one or more waste channels.

The measurement means may comprise a first electrode pair including afirst electrode and a first counter electrode and a second electrodepair downstream from said first electrode pair, the second electrodepair comprising a further electrode and a further counter electrode,wherein the first and further electrodes are arranged to be coupled tothe same current signal and the first and further counter electrodes arearranged to be coupled to ground. Such an electrode arrangement issuitable for performing an impedance measurement such as a white bloodcell count. The electrodes preferably are platinum electrodes.

The measurement chip of the present invention may be integrated into amicrofluidic device that further comprises a microfluidic resistancenetwork comprising a microfluidic sample preparation stage, a sampleoutput and a waste output both in fluidic communication with saidpreparation stage, wherein the sample channel is in fluidiccommunication with said sample output and the waste channel is influidic communication with said waste output.

Such a microfluidic device exhibits improved controllability over theaccuracy of the tuning of the flow rates through the device, therebyimproving the accuracy of the measurement results obtained with themicrofluidic device.

The microfluidic resistance network is preferably made of a polymermaterial as this allows for a cost-effective manufacturing of themicrofluidic resistance network with good control over the dimensions ofthe microfluidic network.

In a preferred embodiment, the microfluidic device further comprises afirst inlet for receiving a sample; a second inlet for receiving adiluent; and wherein the sample preparation stage comprises a chain ofdilution stages for diluting said sample with said diluent in which adownstream dilution stage is arranged to further dilute the samplereceived from the previous dilution stage in said chain, wherein thefirst (36) of said dilution stages is in fluidic communication with thefirst inlet (22′); each of said dilution stages is in fluidiccommunication with the second inlet (24); and at least some of thedilution stages comprise a separate output (43, 44) for feeding part ofthe received sample to the waste channel (114).

The present invention is particularly applicable to microfluidicresistance networks in which a sample is heavily diluted by feeding adiluent from a single inlet to a plurality of dilution stages, as insuch an arrangement a large portion of the sample is typically fed towaste, as in each dilution stage only a fraction of the incoming sampleis combined with the diluent, with the remainder of the incoming samplebeing discarded.

In a preferred embodiment, the sample is a blood sample and themicrofluidic resistance network further comprises a further samplepreparation stage; a first further inlet in fluidic communication withthe further sample preparation stage for receiving the blood sample; asecond further inlet in fluidic communication with the further samplepreparation stage for providing a lysis agent to the further samplepreparation stage; a third further inlet in fluidic communication withthe further sample preparation stage for providing a quench solution tothe further sample preparation stage and a further sample outlet influidic communication with the further sample preparation stage.

Such a microfluidic device can for instance be used as a single devicefor performing a FBC, with the sample preparation stage comprising thechain of dilution stages preparing part of the blood sample for aRBC/platelet analysis and the further sample preparation stage preparinganother part of the blood sample for a WBC count. To this end, themicrofluidic device preferably further comprises an optical measurementcell for measuring a haemoglobin count. This optical measurement cellmay be in fluidic communication with the first inlet via the samplepreparation stage, the further sample preparation stage or may be fedoff a separate blood sample inlet.

In accordance with another aspect of the present invention, there isprovided a method of manufacturing measurement chip for use with amicrofluidic resistance network comprising a microfluidic samplepreparation stage, a sample outlet and a waste outlet both in fluidiccommunication with said dilution stage, the method comprising providinga glass substrate; forming a sample channel through said glasssubstrate; forming a waste channel through said glass substrate; andforming measurement means in said sample channel.

BRIEF DESCRIPTION OF THE EMBODIMENTS

Embodiments of the invention are described in more detail and by way ofnon-limiting examples with reference to the accompanying drawings,wherein:

FIG. 1 schematically depicts a microfluidic device;

FIG. 2 schematically depicts an impedance measurement chip and thesignal produced in such a chip;

FIG. 3 schematically depicts an embodiment of a measurement chip of thepresent invention;

FIG. 4 schematically depicts another embodiment of a measurement chip ofthe present invention;

FIG. 5 schematically depicts yet another embodiment of a measurementchip of the present invention;

FIG. 6 schematically depicts a microfluidic device according to anembodiment of the present invention;

FIG. 7 depicts the simulated variation in flow rate through keycomponents of a microfluidic network as a function of measurement chipresistance in a prior art microfluidic device; and

FIG. 8 depicts the simulated variation in flow rate through keycomponents of a microfluidic network as a function of measurement chipresistance in a microfluidic device according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE DRAWINGS

It should be understood that the Figures are merely schematic and arenot drawn to scale. It should also be understood that the same referencenumerals are used throughout the Figures to indicate the same or similarparts.

The present invention relates to microfluidic devices that comprise aplurality of discrete components, in particular a microfluidicresistance network, which may be in the form of a disposable cartridge,and a measurement chip. The microfluidic resistance network has thepurpose of sample preparation and presenting the prepared sample to themeasurement chip. In the context of the present invention, the term‘microfluidic’ is to relate to the behavior, precise control andmanipulation of fluids that are geometrically constrained to a small,typically sub-milliliter, scale volumes, e.g. μl, nl, pl, fl volumes.

FIG. 1 schematically depicts a non-limiting example of such amicrofluidic device 10, which includes a disposable microfluidicresistance network 20 and a measurement chip 50. The microfluidicresistance network 20 is designed to receive a sample such as a RBCsample at sample inlet 22. The microfluidic resistance network 20further comprises a diluent inlet 24 for receiving a diluent, which isbranched off in three different branches. A first branch is mixed withthe sample at the sample inlet 22 and subsequently fed to a samplemixing or dilution stage 34, e.g. a snake stage, whereas a second branchis used to further dilute the sample at junction 36. The junction 36 istypically shaped in a particular manner to obtain a desired dilutionratio of the sample with the diluent, as for instance is explained inmore detail in WO 2010/086786. A sample dilution stage 38, e.g. amicrofluidic snake stage, is designed such that that the sample is incontact with the diluent for a predetermined period of time, e.g. aperiod of time necessary to complete the dilution of the sample and toprovide the required fluidic resistance At junction 36, a substantialpart of the diluted sample received from dilution stage 34 is fed to awaste channel 43, whereas a (small) fraction of the diluted sample ismixed with the diluent from the second branch of the diluent inlet 24and fed to the sample dilution stage 38. At junction 40, the samplediluted in sample dilution stage 38 is again split in a portion fed towaste channel 44, with the remaining portion further diluted by thediluent received from the third branch of the diluent inlet 24 andsubsequently fed via measurement channel 42 to the measurement chip 50.A snake stage (not shown) may be present between the junction 40 and themeasurement chip 50 for the aforementioned reasons. Junction 40 istypically shaped in a particular manner to obtain a desired dilutionratio of the diluted sample received from the sample dilution stage 34with the diluent. Suitable embodiments of diluent have for instance beendisclosed in WO 2010/086786.

The microfluidic device 10 shown in FIG. 1 is particularly suited forthe treatment and subsequent analysis of a FBC sample. However, it willbe understood by the skilled person that design of the microfluidicresistance network 20 may be altered to prepare different types ofsamples, e.g. urine or saliva samples, as well as samples fornon-medical evaluation, e.g. environmental samples, food samples and soon.

FIG. 2 shows the impedance measurement chip 50 in more detail. Adetailed description of such an impedance measurement arrangement can befound in “Impedance spectroscopy flow cytometry: on-chip label-free celldifferentiation”, Cheung, K., S. Gawad, and P. Renaud, Cytometry A,2005. 65(2): p. 124-132. FIG. 2 shows a side view of the microfluidicchannel through the chip 50 and a sample cell 80 passing between theexcitation electrodes 52, 62 and the detection electrodes 54, 64. Theexcitation electrode 52 and the detection electrode 54 form a firstelectrode pair and the excitation electrode 62 and the detectionelectrode 64 form a second electrode pair.

The excitation electrodes 52 and 62 are respectively connected to acurrent input signal source 58 and 68, e.g. an AC or DC input signalsource. An AC input signal source is preferred as it preventselectrolysis at the electrodes. In an embodiment, the excitationelectrodes 52 and 62 may share the same AC input signal source (i.e.58=68). The detection electrodes are typically connected to adifferential potential detection circuit 70, which preferably keeps thedetection electrodes at an approximate ground potential. The currentspassing through the fluid between the first and second electrode pairare amplified and its difference is determined in any suitable manner,e.g. using well-known analog electronics. The in-phase and out-of-phaseparts of the resulting AC signal are measured using standardLock-in-technology. Without a particle passing the electrodes themeasured signal is ideally zero, although in practice always an offsetis present due chip asymmetry, and potentially electronic componentinaccuracies. If a particle coming from the left first passes the firstelectrode pair, a positive almost Gaussian shape signal is produced, asthe second electrode pair acts as the reference electrode for the firstelectrode pair. When the particle subsequently passes the secondelectrode pair, a negative Gaussian shape signal is produced as thefirst electrode pair acts as the reference electrode for the secondelectrode pair. The resulting antisymmetric double Gaussian signal shapeis also shown in FIG. 2. The Cell Signal may be the output of a Lock-InAmplifier measuring the current difference between both electrode pairs.In this way impedance spectroscopy can be performed for different cells,e.g. RBC or WBC.

In order to accurately determine the cell count of the sample fedthrough the measurement chip 50, the flow rate of the sample through themeasurement chip 50 must be well defined, as the number of counted cellsmust be related to a well-defined sample volume. This sample volume isset by junction 40 of the microfluidic resistance network 20, and isinfluenced by the respective fluidic resistances of the microfluidicpath through the measurement chip 50 and the microfluidic waste channel44. In order to correctly design the junction 40, these fluidicresistances must be known at the design stage. As the hydraulic diameterof the microfluidic channel through the measurement chip 50 aretypically much smaller than the hydraulic diameter of the microfluidicwaste channel 44, the microfluidic waste channel 44 typically comprisesa matching element (not shown) to match the fluidic resistance of themicrofluidic waste channel 44 with that of the microfluidic channelthrough the measurement chip 50. The hydraulic diameter is the effectivediameter used to approximate the behaviour of a non-circular channel,such as a square or rectangular channel.

However, as the measurement chip 50 is manufactured in a differentmanufacturing process than the microfluidic resistance network 20, themanufacturing process of the measurement chip 50 is typically subjectedto different tolerances than the manufacturing process of themicrofluidic resistance network 20. As previously mentioned, theresolution of the microfluidic resistance network 20 is typically chosento be lower than the resolution of the measurement chip 50. For thisreason, the (disposable) microfluidic resistance network 20 can becheaply manufactured in polymer materials, e.g. plastics, whereas forthe measurement chip 50 glass substrates are preferred as glass is arelatively cheap material suitable for achieving the requiredresolution.

The consequence of the use of these different manufacturing processesand different materials for the microfluidic resistance network 20 andthe measurement chip 50 is that the tolerances in these processes andmaterials can lead to an unacceptably large deviation of the junction 40from its intended performance, e.g. the volume of the sample separatedfrom the waste stream is inaccurate. This is typically caused byindependent variations in the dimensions of the microfluidic wastechannel 44 and the microfluidic channel through the measurement chip 50.

FIG. 3 shows a measurement chip 100 in accordance with an embodiment ofthe present invention in which this problem has been overcome. Inaddition to the microfluidic measurement channel 104 extending betweeninlet 102 and outlet 106, the measurement chip 100 further comprises amicrofluidic waste channel 114 extending between inlet 112 and outlet116 for receiving the remainder of the sample from microfluidic wastechannel 44. The microfluidic measurement channel 104 may comprise anysuitable measurement means, e.g. an electrode arrangement such as theimpedance measuring first electrode pair 120 and a second electrode pair130 as previously explained with the aid of FIG. 2, or optical detectionmeans, e.g. a spectrophotometer, photodiode and so on.

Any suitable electrode material may be chosen. Preferably, theelectrodes are platinum electrodes although alternative electrodematerials such as for example Indium Tin Oxide (ITO), titanium nitrideand chromium nitride are also feasible. It should further be understoodthat the electrode arrangement shown in FIG. 2 has been shown by way ofnon-limiting example only, and that other electrode arrangements areequally feasible, such as a single pair of electrodes or a long sequenceof electrode pairs as disclosed in WO2010/086797 A1. The electrode maybe controlled by any suitable control signal, e.g. an AC signal aspreviously explained as well as a DC signal.

As the hydraulic diameter of the microfluidic waste channel 114 is muchsmaller than the hydraulic diameter of the microfluidic waste channel44, the fluidic resistance of the combined waste line is entirelydominated by the fluidic resistance of the microfluidic waste channel114, such that tolerances in the manufacturing process of themicrofluidic resistance network 20 no longer affect the sampleseparation ratio at junction 40. This sample separation ratio is nowdominated by the fluidic resistances of the microfluidic measurementchannel 104 and the microfluidic waste channel 114 respectively. As themicrofluidic measurement channel 104 and the microfluidic waste channel114 are formed in the same manufacturing process, the tolerances in thedimensions of the microfluidic measurement channel 104 and themicrofluidic waste channel 114 can be much better controlled, such thatthe volume of the sample directed towards the microfluidic measurementchannel 104 at junction 40 becomes invariant of the manufacturingtolerances of both the microfluidic resistance network 20 and themeasurement chip 100.

At this point, it is noted that although extending the measurement chip100 with the microfluidic waste channel 114 increases the cost of themeasurement chip 100, the overall cost of a microfluidic deviceincluding a microfluidic resistance network 20 and the measurement chip100 is reduced as the number of incorrectly tuned devices as caused byoverly large variations in manufacturing tolerances is significantlyreduced.

The cost of the measurement chip 100 may be reduced by combination ofthe outlets of the measurement channel 104 and the microfluidic wastechannel 114 as shown in FIG. 4. In FIG. 4, outlet 116 has been removedfrom the design of the measurement chip 100. It has been found that thecombination of multiple microfluidic channels on a single outlet doesnot influence the respective microfluidic resistances of the multiplemicrofluidic channels.

Preferably, the microfluidic measurement channel 104 and themicrofluidic waste channel 114 have the same dimensions, as the fluidicresistance R of a microfluidic channel scales as follows: R˜1/d⁴, inwhich d is the hydraulic diameter of the microfluidic channel. It hasbeen found that for microfluidic channels having different dimensions,i.e. different hydraulic diameters, different variations in thetolerances of these channels can occur, which makes it more difficult totune the microfluidic device and in particular the microfluidicresistance network 20 including junction 40.

If nevertheless the microfluidic measurement channel 104 requires adifferent fluidic resistance than the microfluidic waste channel 114,this preferably is achieved by providing a plurality of microfluidicchannels for the channel requiring the lower resistance. Each of themicrofluidic channels in said plurality preferably have the samehydraulic diameter for the reasons already explained above. Anon-limiting example of such an arrangement is shown in FIG. 5, in whichin addition to the first microfluidic waste channel 114, a secondmicrofluidic waste channel 114′ between inlet 112′ and outlet 116′ isprovided, with the waste stream from microfluidic waste channel 44 beingequally divided over the first microfluidic waste channel 114 and thesecond microfluidic waste channel 114′.

It will be understood that many variations are possible to themeasurement chip 100 shown in FIG. 5. In FIG. 5, two microfluidic wastechannels 114 and 114′ are shown by way of non-limiting example only. Adifferent number of microfluidic waste channels is equally feasible;typically the measurement chip 100 comprises N microfluidic wastechannels, with N being a positive integer if the ratio between themicrofluidic resistance of the microfluidic measurement channel 104 andthe microfluidic resistance of the microfluidic waste channels equals N(R₁₀₄/R_(114, 114′)=N). Although the two microfluidic waste channels 114and 114′ are shown to have separate inlets and outlets, it will beunderstood that the microfluidic waste channels 114 and 114′ may sharetheir inlets and/or outlets, or may share their outlets with themicrofluidic measurement channel 104. It will further be understood thatinstead of or in addition to having a plurality of microfluidic wastechannels, the measurement chip 100, microfluidic measurement channel 104may also comprise one or more dummy channels preferably having the samedimensions as the microfluidic measurement channel 104 to lower thefluidic resistance of the microfluidic measurement channel 104. Themeasurement chip 100 may for instance comprise a microfluidicmeasurement channel 104 and M-1 dummy channels (not shown) as well as Nmicrofluidic waste channels in case the desired ratio of the respectivefluidic resistances is M/N. In addition, the measurement chip 100 maycomprise a plurality of different measurement channels for measuringdifferent samples or different portions of the same sample, e.g.different portions treated with different reagents to measure differentaspects of the same sample, e.g. separate channels for performing a RBCand a WBC count.

The measurement chip 100 may be manufactured in any suitable manner, forinstance by providing a top plate and a bottom plate, preferably glassplates, forming a pair of trenches in each of the top plate and thebottom plate, e.g. by etching or drilling such as laser drilling,forming measurement means in one of said trenches in the top plate andin the corresponding trench in the bottom plate, and placing the topplate onto the bottom plate such that a first pair of trenches combineto form the microfluidic measurement channel 104 including themeasurement means and a second pair of trenches combine to form themicrofluidic waste channel 114. Alternative manufacturing methods willbe apparent to the skilled person, e.g. by providing a substrate,preferably glass, drilling the microfluidic measurement channel 104 andthe microfluidic waste channel 114 in the substrate, e.g. by laserdrilling and forming the measurement means in the microfluidicmeasurement channel 104.

FIG. 6 schematically depicts an embodiment of a microfluidic device 200of the present invention. The microfluidic device 200 is designed toperform a FBC on a single blood sample. To this end, the microfluidicdevice 200 comprises a first blood sample input 22 into a lysis stagefor lysing red blood cells, including an inlet 25 for receiving a RBClysing agent such as a formic acid/saponin mixture, and an inlet 26 forreceiving a quenching agent for quenching the lysed sample to protectthe white blood cells from lysing. A non-limiting example of a suitablequenching agent is a NaCl/NaHCO3 solution. The lysing stage may compriseany suitable number of snake stages. Two snake stages 35 and 37 areshown by way of non-limiting example. The outlet channel 46 of thelysing stage is fed into the measurement chip 100 of the presentinvention. As the entire sample is fed through the measurement chip 100,there is no separate waste channel through the chip 100 for the lysisstage.

The microfluidic device 200 further comprises a second blood sampleinlet 22′, which is fed into a red blood cell/platelet treatment stage.The first blood sample inlet 22 and the second blood sample inlet 22′may be separate branches of a single blood sample inlet (not shown) ormay be independently fed with separate blood samples, e.g. separateportions of the same blood sample. The blood cell/platelet treatmentstage further comprises a diluent sample inlet 24, which is split inthree branches. A first branch is fed to the blood sample inlet 22′where the incoming blood sample is diluted by a predefined ratio, e.g.20:1, and the second and third branch are fed to junctions 36 and 40respectively, where the diluent is mixed with blood sample.Consequently, large dilution ratios can be achieved with only a smallamount of diluent as no diluent is wasted in the microfluidic device200.

Each of the junctions 36 and 40 has a first output for generating awaste stream essentially comprising of a large fraction of the incomingsample only, and a second output for generating a mixture of the smallfraction of the incoming sample with all of the incoming diluent. Thevarious fluidic channels may contain one or more snake stages, e.g.stages 34 and 38, which may be included to tune the mixing ratio and thefluidic resistance of the fluidic channel, as is known per se. Thesample output 42 of the junction 40 is fed to the sample channel of themeasurement chip 100, e.g. the measurement channel 104 as for exampleshown in FIG. 3 for measuring a red blood cell count, whereas the wasteoutput 44 the junction 40 is fed to the waste channel of the measurementchip 100, e.g. the waste channel 114 as for example shown in FIG. 3, forreasons previously explained. In an embodiment, the waste streams of therespective dilution stages, i.e. junctions 36, 40 may be combined priorto feeding the waste through the waste channel 114 of the measurementchip 100. This is shown in FIG. 6 as the microfluidic branch labelled43+44.

Compared to the microfluidic resistance network 20 in the microfluidicdevice 10, the microfluidic resistance network 20 of the microfluidicdevice 200 can be simplified by the omission of the matching element inthe (combined) microfluidic waste channel (43+) 44, as this resistancematching is now achieved by feeding the waste through the higherresolution (i.e. smaller dimensioned) microfluidic waste channel 114.

In FIG. 6, the waste channel 43 from the first junction 36 is branchedoff towards to a Hb sample chamber 230 including an optical measurementcell for preparing the unused portion of the blood sample for a Hbabsorption measurement in the optical measurement cell in the Hb samplechamber 230. The Hb sample chamber may contain some reagents in dry formthat lyse and label the blood sample to perform the Hb measurement. Inthis arrangement, only a small sample of blood needs to be labeled forthe Hb absorption measurement, which is advantageous as the labelingreagents can be toxic, e.g. comprise cyanide, as necessarily they mustbind to Hb.

It is pointed out that FIG. 6 shows a non-limiting example of amicrofluidic device 200 of the present invention. The microfluidicdevice 200 may for instance be a microfluidic device as described indetail in WO2010/086786. In FIG. 6, a part of the sample is branched offthe RBC count preparation stage for Hb measurement preparation in thepreparation stage 230. It will be understood that it is equally feasibleto branch off part of the sample from the WBC count preparation stagefor Hb sample preparation instead. The microfluidic device 200 mayalternatively be arranged to generate three separate branches from theblood sample inlet 22, i.e. one branch for RBC/platelet count samplepreparation, one branch for WBC sample preparation and one branch for Hbmeasurement sample preparation. Other variations will be apparent to theskilled person.

It will further be understood that the present invention is not limitedto microfluidic devices for FBC measurements. The present invention maybe applied to any microfluidic device in which a tuned microfluidicresistance network 20 is manufactured separate from a measurement chip100, and in which only a fraction of the sample prepared in the tunedmicrofluidic resistance network 20 is to be fed to the measurement chip100, e.g. microfluidic devices for analysis of bodily fluids such assaliva and urine, microfluidic devices for analysis of environmentalsamples, food samples and so on.

FIG. 7 depicts the result of a simulation of the flow rates through keycomponents of the tuned microfluidic resistance network 20 in themicrofluidic device 10 of FIG. 1 as a function of the fluidic resistanceof the microfluidic measurement channel of the measurement chip 50. InFIG. 7, the thin solid line is the flow rate from junction 40 to thewaste outlet 44, the thick solid line is the flow rate of the sampleinto junction 40, the dashed line (---) is the flow rate from junction40 towards the sample outlet 42 and the chain line (-•-•) is the flowrate of the diluent into the junction 40. As the waste stream fromjunction 40 is not fed through the measurement chip, these flow ratesare affected by a change in the fluidic resistance of the microfluidicmeasurement channel of the measurement chip 50. Importantly, as the flowrates change, the dilution ratios change as well such that the absolutecell counts per unit of the sample are no longer (accurately) known. Itwill be apparent that such variations in flow rate are unacceptable.

FIG. 8 depicts the result of a simulation of these flow rates throughthe tuned microfluidic resistance network 20 in the microfluidic device200 of FIG. 6 as a function of the fluidic resistance of themicrofluidic measurement channel 104 of the measurement chip 100. As thewaste stream from junction 40 is fed through the microfluidic wastechannel 114 of the measurement chip 100, these flow rate are no longerdependent of changes in the fluidic resistance of the microfluidicmeasurement channel 104 of the measurement chip 100, due to the factthat the fluidic resistance of the microfluidic waste channel 114 variesin the same manner. This clearly demonstrates that the provision of thewaste channel on the measurement chip 100 significantly improves therobustness of the overall microfluidic device 200 against variations(tolerances) in the manufacturing process of its components.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention can be implemented by means of hardware comprising severaldistinct elements. In the device claim enumerating several means,several of these means can be embodied by one and the same item ofhardware. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage.

The invention claimed is:
 1. A measurement chip for use with amicrofluidic resistance network for measuring parameters in a full bloodcount test, the measurement chip comprising: a sample channel forproviding a flow of a sample stream in one direction only from a sampleinlet to a sample outlet, the sample inlet receiving the sample streamfrom the microfluidic resistance network, the sample channel having afirst fluidic resistance; and a waste channel for providing a flow of awaste stream in one direction only from a waste inlet to a waste outlet,the waste inlet receiving the waste stream from the microfluidicresistance network, the waste channel having a second fluidicresistance, wherein a ratio between the first fluidic resistance and thesecond fluidic resistance is predetermined.
 2. The measurement chip ofclaim 1, wherein the respective dimensions of the sample channel and thewaste channel are the same.
 3. The measurement chip of claim 2, whereinthe sample outlet and the waste outlet are the same.
 4. The measurementchip of claim 1, wherein the chip comprises a plurality of said wastechannels.
 5. The measurement chip of claim 4, wherein said wastechannels share the same waste inlet and/or the same waste outlet.
 6. Themeasurement chip of claim 1, wherein the chip is a glass chip.
 7. Themeasurement chip of claim 1, further comprising a first electrode pairincluding a first electrode and a first counter electrode, themeasurement chip further comprising a second electrode pair downstreamfrom said first electrode pair, the second electrode pair comprising asecond electrode and a second counter electrode, wherein the first andsecond electrodes are configured to be coupled to the same currentsignal, and the first and second counter electrodes are configured to becoupled to ground.
 8. The measurement chip of claim 7, wherein saidelectrodes and counter electrodes are platinum electrodes.